People often ask about the feasability of building a DIY solution given the $800 cost of the commercial units.
Well, here's a teardown of a commercial Eno Scientific "Well Watch 670". It uses a regular mid range speaker as the sound maker and a two wire microphone for the receiver. The mechnical arrangements of the ports along with a plug in extension tube for the microphone is probably the most interesting design aspect.
The rest of the magic is in the signal processing. I don't know what hardware they used 20+ years ago, but given the fast microcontrollers available today, this is probably a doable project for someone who knows how to use an oscilloscope.
